The present invention relates to the field of fluid dynamic bearing assemblies, and more specifically to improved apparatus for sealing the improved fluid dynamic bearing from the outside atmosphere.
Disc drive memory systems have been used in computers for many years for storage of digital information. Information is recorded on concentric memory tracks of a magnetic disc medium, the actual information being stored in the form of magnetic transitions within the medium. The discs themselves are rotatably mounted on a spindle, the information being accessed by means of transducers located on a pivoting arm which moves radially over the surface of the disc. The read/write heads or transducers must be accurately aligned with the storage tracks on the disc to ensure proper reading and writing of information; thus the discs must be rotationally stable.
During operation, the discs are rotated at very high speeds within an enclosed housing by means of an electric motor which is generally located inside the hub or below the discs. One type of motor in common use is known as a spindle motor. Such motors typically have a spindle mounted by means of two ball bearing systems to a motor shaft disposed in the hub. One of the bearings is typically located near the top of the spindle, and the other near the bottom. These bearings allow for rotational movement between the shaft and hub, while maintaining accurate alignment of the spindle to the shaft. The bearings themselves are normally lubricated by grease or oil.
The conventional bearing system described above, however, is prone to several shortcomings. First is the problem of vibration generated by the balls rolling on the raceways. Ball bearings used in hard disc drive spindles run under conditions that generally result in physical contact between raceway and ball, in spite of the lubrication layer provided by the bearing oil or raceway and ball, in spite of the lubrication layer, in spite of the lubrication layer provided by bearing oil or grease. Hence, bearing balls running on the generally smooth but microscopically uneven and rough raceways transmit this surface structure as well as their imperfections in sphericity in the form of vibration to the rotating disc. This vibration results in misalignment between the data tracks and the read/write transducer, limiting the data track density and the overall performance of the disc drive system.
Another problem is related to the application of hard disc drives in portable computer equipment and resulting requirements in shock resistance. Shocks create relative acceleration between the discs and the drive casting which in turn show up as a force across the bearing system. Since the contact surfaces in ball bearings are very small, the resulting contact pressures may exceed the yield strength of the bearing material, and leave long term deformation and damage to the raceway and the balls of the ball bearing.
Moreover, mechanical bearings are not easily scaleable to smaller dimensions. This is a significant drawback since the tendency in the disc drive industry has been to continually shrink the physical dimensions of the disc drive unit.
As an alternative to conventional ball bearing spindle systems, hydrodynamic bearings are being adopted. In these types of systems, lubricating fluidxe2x80x94either gas or liquidxe2x80x94functions as the actual bearing surface between a stationary base or housing and the rotating spindle or rotating hub of the motor. For example, liquid lubricants comprising oil, more complex ferro-magnetic fluids or even air have been utilized in hydrodynamic bearing systems. The reason for the popularity of the use of air is the importance of avoiding the outgassing of contaminants into the sealed area of the head/disc housing. However, air does not provide the lubricating qualities of oil. The relatively high viscosity of oil allows for larger bearing gaps and therefore greater tolerances to achieve similar dynamic performance.
An essential feature of such fluid dynamic bearings is to seal the bearing from the surrounding atmosphere, especially when the bearing or the motor in which the bearing is incorporated is to be used in a disc drive. In the most common form of hydrodynamic bearing, a shaft is provided, supported for relative rotation relative to a surrounding sleeve and for radial stiffness by a journal bearing. Such a bearing typically includes two sets of grooves defined on either the external surface of the shaft or the internal surface of the sleeve. Fluid in the gap between shaft and sleeve supports the relative rotation. Typically, a capillary seal is defined by a gradual divergence of one of the walls of the gap away from the other wall is used at an end of the journal bearing to prevent loss of any fluid. This seal design, located at an end of the journal bearing along the sleeve bore, is designed to prevent fluid leakage outwardly during operation, and also to prevent the intake of ambient air when the system operates at vacuum or under low internal pressure. However, the use of this design limits the journal bearing span from being enlarged; and when the shaft is shortened for a small form factor bearing or motor design, it makes balancing the bearing more difficult.
Therefore, an alternative design which allows for enlargement of the journal bearing span, as well as balancing the bearings closer about the center of the rotating shaft or closer to the overall center of gravity of the design is highly desirable.
The present invention is intended to provide advantages over and solve many of the problems of the prior art.
In one embodiment, the invention allows more room for the journal bearing to be extended along the bore between the shaft and the sleeve.
The present invention further is intended to increase bearing stiffness, by allowing for extension of the journal bearing along the shaft and the sleeve.
The present invention further has the objective of allowing modified placement of the journal bearing grooves along the shaft and sleeve to allow improved balancing of the bearing locations relative to the center of gravity of the motor and/or disc drive in which the bearing is included, as well as improved balance of the bearing system in which the seal is included.
These and other objectives and advantages are achieved in an unvented design comprising a fluid dynamic bearing including a shaft rotating within a sleeve. The shaft and sleeve are supported for relative rotation by fluid in the gap between the shaft and sleeve. To prevent fluid from leaking out of the fluid dynamic bearing into the surrounding atmosphere, one end of the fluid is defined by a capillary seal.
According to the present invention, rather than have the capillary seal defined between the shaft and sleeve, the seal is defined between the hub which rotates to establish rotation of the discs, and a facing surface of the sleeve. The surface of the hub and surface of the sleeve are in relative rotation, and diverge from one another in the region where the capillary seal is to be established. The gap which is incorporated in the capillary seal is in airway communication with the gap between the shaft and the sleeve. But the capillary seal is now defined by a radially outer surface of the sleeve and is substantially parallel to the gap between the shaft and the sleeve.
In a preferred embodiment, the capillary seal is substantially parallel to but radially spaced from the region where one of the dual journal bearing groove sets is defined.
In a further preferable approach, the grooves on the sleeve that define the dual journal bearings are located so that they are distributed as well as possible on either side of the center of gravity of the system in which the journal bearing and capillary seal combination is located. Other features and advantages of the invention will be apparent to a person of skill in this field who studies the description of the following preferred embodiment given with respect to the following drawings.